Gyrocompass follow-up system



United States Patent 3,443,321 GYROCOMPASS FOLLOW-UP SYSTEM Werner Auer,Heidelberg-Wieblingen, Germany, assignor to Teldix Luftfahrt-AusrustungsG.m.b.H., Heidelberg- Wieblingen, Germany Original application Mar. 11,1965, Ser. No. 438,977, now Patent No. 3,386,179, dated June 4, 1968.Divided and this application Apr. 13, 1967, Ser. No. 630,585 Claimspriority, application Germany, Apr. 24, 1964, T 26,070 Int. Cl. G01c19/38 US. Cl. 33226 5 Claims ABSTRACT OF THE DISCLOSURE A gyrocompassrotor frame is mounted on a stable vertical-axis low-friction airbearing. It is torqued azimuthally by springs which also serve aselectrical connections between the rotor frame and an enclosing casingrotatable in azimuth by a follow-up motor controlled by a pick-offbetween the casing and the rotor frame. The pick-off signalamplification is, for the first part of the meridian seeking swing, madesuch as to damp the oscillatory transients most rapidly; and when thepick-off signal, or alternatively the amplifier output, has fallen belowa predetermined value, a program of increased amplification isautomatically initiated.

REFERENCE TO RELATED APPLICATION This is a division of application Ser.No. 438,977, filed Mar. 11, 1965, now Patent No. 3,386,179 dated June 4,1968.

BACKGROUND OF THE INVENTION The present invention relates to agyrocompass, and particularly to a follow-up system for connectionbetween the rotor element and the casing of such gyrocompass.

A conventional gyrocompass incorporates a one-degreeof-freedom gyrorotor in which a first frame or gimbal carrying the rotor is rotatablewith respect to a second frame or housing, the first frame beingrotationally elastically coupled to the second frame by means of atorque generator or so-called torquer. The second frame is rotatablewith respect to the earth about a vertical axis and the first framerotates within the second frame about the same axis. The gyro rotor isso mounted in the first frame that the spin axis of the rotor is alwaysat right angles to the axis of the frame, i.e., that the spin axis isalways horizontal to the same extent that the frame axis is vertical.Suitable electric read-out or so-called pick-off means are providedwhich produce a signal that is a function of the angle between the twoframes, and the value read-out is applied to the torquer in such amanner as to produce a torque which acts against an increase of theangle. In this way, the pick-off means and the torquer form a systemwhich in effect spring-couples the two frames to each other. If noexternal forces act on the frames, they will assume a definite angularposition with respect to each other.

Inasmuch as, due to the horizontal component of the earths rotation, therotor has the tendency to align its spin axis with the north-southdirection, the gyrocompass can be used to find the true or polar northby manually turning the second frame until it coincides with the firstframe, i.e., until the second frame assumes the quiescent position ofcoincidence between the spin axis and the housing spin reference axisdictated by the abovementioned pick-off means and torquer.

Another type of conventional gyrocompass, which operates on essentiallythe same principle, additionally ice includes a follow-up system whichconsists of an amplifier and a servomotor that drives the second frameabout its axis. So long as the two frames form an angle with each other,the servomotor will drive the second frame in such a direction as toreduce the angle. After the oscillations have decayedwhich delays thetime until the compass accurately indicates true norththe two frameswill assume their rest positions. As in the case of the first-describedgyrocompass, the direction of the spin avis of the rotor is indicated bymeans of the second frame, or is otherwise suitably utilized as atrue-north reference.

SUMMARY OF THE INVENTION It is therefore a primary object of the presentinvention to provide a gyrocompass having a follow-up system whichovercomes the above-noted drawbacks.

Another object of the present invention is to provide a gyrocompasshaving a very low warm-up time so that it indicates the north-southdirection shortly after being put into operation.

The follow-up system according to the present invention may be providedin a gyrocompass of the type described in the above-mentioned Patent No.3,386,179. Such a gyrocompass is, basically, a one-degree-of-freedomgyrocompass in which the first frame, i.e., the frame which supports thegyro rotor and its electric drive motor, is in the form of a cylinderand the second frame is in the form of a cup-shaped housing, thecylinder being mounted in the cup-shaped housing for rotation about thevertical axis, and there being air-cushioning between the cylinder andhousing so that a so-called gas bearing is formed. Furthermore,mechanical spring elements are provided which provide an elasticcoupling acting against the rotation of the two frames relative to eachother about the vertical axis, and these mechanical spring elementsfurther serve as the current lead-in means by which electrical energy issupplied to the gyro rotor drive motor from an external power source. Inpractice, the spring means may also constitute the electrical connectionbetween a pick-off component that is movable with the first frame andthe remainder of a follow-up system, the latter being interposed betweenthe two frames and responsive to the angle formed between them forapplying to the second frame a force proportional to the angle betweenthe frames for aligning the second frame with the northsouth direction.

In accordance with the present invention, the followup system which isinterposed between the first frame and the gyrocompass casingincorporates an amplifier for feeding a servo motor which is arranged todrive the casing in such a direction as to align a diametral plane ofthe casing with the rotor spin axis. This followup system differsessentially from conventional prior art follow-up amplifiers as follows:the amplifier produces a so-called aperiodic amplification V which, aswill be described in more detail below, is the value that will cause theangle a between the rotor element and the gyrocompass casing to becomestabilized as quickly as possible. However, this angle produces acertain north error," and this error can be substantially reduced if theamplification factor is greater than V Such higher amplification,however, would materially increase the time it takes for the angle a tobecome stabilized, i.e., the warm-up time of the gyrocompass. Accordingto a further feature of the present invention, therefore, the amplifieris a variable one and, while initially set to produce the aperiodicamplification V,,,,, is adjusted to assume a higher amplification valueafter the angle on has reached a predetermined minimum value. In thisway, the north error is reduced without, however, increasing the warm-uptime of the gyrocompass.

3 BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a vector diagram showingthe relevant axis and angles between the various elements of thegyrocompass system.

FIGURE 2 is a block diagram of a gyrocompass system, showing theinteraction of the various forces and components.

FIGURE 3 is a graph showing various characteristics of the system.

FIGURE 4 is a block diagram of one embodiment of the amplifier controlsystem according to the present invention for increasing theamplification to decrease the north error.

FIGURE 5 is a block diagram of another embodiment of the amplifiercontrol system for increasing the amplification and hence decreasing thenorth error.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is often assumed, as a roughapproximation, that the spin reference axis of the gyrocompass casingsurrounding the gyrocompass rotor and first frame will, after thewarm-up time of the system, orient itself precisely to the north-southdirection. However, as will now be explained in conjunction with FIGURES1, 2 and 3, the spin reference axis of the gyrocompass casing will, inpractice, assume an orientation which differs slightly from the truenorth-south direction. The magnitude of this error angle, as thedifference between the true north-south direction and the directionactually assumed by the spin reference axis of the casing may be termed,can not be predicted with accuracy, inasmuch as the rotational speed ofthe compass as it warms up and ultimately assumes its rest position canvary within certain limits, and the precise speed will be determined b anumber of random factors. Hereinafter, reference will be made to thegreatest possible error angle 7, namely '7 which will hereinafter betermed simply as the north error.

In FIGURE 1, the arrow 60 represents the true or geographic northdirection, and can therefore be considered to be a fixed direction.Arrow 61 represents the spin axis of the north-seeking gyro rotor. Thisrotor can turn only about this spin axis and about a further axis 62,namely, the vertical axis which passes through the center of the earthand which is perpendicular to the plane of the drawing. The spinreference axis of the gyrocompass casing, represented by arrow 63, isalso rotatable about the vertical axis 62. The casing is, as describedabove, elastically connected with the spin axis. This elastic connectionhas the effect that when the spin axis and spin reference axis,represented by arrows 61 and 63, form an angle a with each other, amoment M =Da is produced which tends to bring the two arrows intoalignment with each other. Here, D is the directional spring constant ofthe elastic connection. Due to the earths rotation, there is exerted onthe spin axis, which forms an angle with true north, moment M thatdrives the spin axis toward the north direction, as follows:

0=the moment of inertia with respect to the spin axis,

w=the angular speed of the rotor,

w*=tl1e angular speed of the earth,

=the geographic latitude of the location of the gyrocompass,

fl=the angle between the rotor spin (arrow 61 in FIG- URE 1) and thenorth direction (arrow 60 in FIG- URE 1).

As is readily apparent from FIGURE 1,

ot=b'y The follow-u p system associated with the gyrocompass, namely,the pick otf, the electronic amplifier and the servomotor, and anytransmission additionally associated with these components, behaves asan integrator, and causes the casing, as represented by arrow 63, to berotated in the direction of the angle a at a rate which is proportionalto the angle on (see also FIGURE 3).

There is thus obtained the block diagram of FIGURE 2 in which therelationship between the input and output signals is shown within therespective blocks. The arrow in the connecting lines between the blocksshow the direction of the signals, i.e., they identify input and outputsignals. It is assumed, in each case, that the input signals control theoutput signals and that there is no internal feedback within any oneblock, so that each block is a one-way transmission element. The blockdiagram also shows branch points, represented by solid dots, e.g., 64,which indicate that the signal flowing into the junction flows out intomore than one component, each of which receives the same signal.Conversely, a circular junction, e.g., 65, represents an additivejunction, in that the signal flowing out of each such junction is themathematical sum of the signals flowing into the junction.

The components embraced by the dashed rectangle 66 represent a simplenorth-seeking gyrocompass with one degree of freedom and withoutdamping. The block contains the function i.e., the specificnorth-driving moment together with a minus sign. Since, in practice, theangle {3 will be small, sin B can be replaced by {3 (linearizing k as afunction of B), so that The moment M drives the spin axis north aboutthe vertical axis. In accordance with Newtons law,

the angle 6 is also obtained from the north-driving moment in the secondblock by dividing by the moment of inertia o of the first frame androtor with respect to the vertical axis and by integrating twice withrespect to the time t.

The gyrocompass according to the present invention, however, is-asexplained above-elastically coupled with respect to the vertical axis.Thus, the spring moment M must always be added to the north-drivingmoment M This spring moment M is proportional to the angle a. As isapparent from FIGURES 1 and 2, u={3-'y.

In FIGURE 2, the block V represents an amplifier for linearly amplifying0c. The next block represents the servomotor and the transmission andthus acts as integrator which integrates a with respect to time, so thatthere is finally obtained the error angle 7, which, in the ideal case,is equal to 0. The value 7 is then multiplied by --l and the value 'y isadded to B, at 65.

The operation of the system shown in FIGURE 2 may be describedmathematically as follows:

s +Vs =0 This characteristic equation of the system describes theoscillatory behavior. Solving the equation for s gives thecharacteristic value for the system. If the amplification V approachesinfinity, the system acts as if only those components embraced Withinthe rectangle 66 were provided, in which case the oscillations are notdamped and the characteristic values are purely imaginary. Similarly,the characteristic values can be imaginary, but of different amounts, ifV approaches 0. The system is damped only if V has a finite value, inwhich case the characteristic value s will be real.

If the amplification has a definite finite value Ic+D M.

Up to now it was assumed that the characteristic of the follow-upsystem, i.e., the angular velocity dot/dz over a, was a straight linepassing through the origin. In that case, the north error would becomeequal to 0. In practice, however, the follow-up system will have aresponse threshold et which is due mainly to the operation of theservomotor but which may be the result of other causes. What is ofimportance is that it is the presence of this response threshold whichis responsible for the north error, in accordance with the followingequation:

FIGURE 3, in heavy lines, shows the characteristic 67 of a follow-upsystem, and corresponds, approximately, to the rotational speedcharacteristic (rotational speed versus applied voltage) of an electricmotor of the type normally used in this field. The characteristic showsthat the servomotor does not actually respond to the presence of anangle a, i.e., that the motor will not rotate, in either direction,until the angle is at least equal to d If, however, the amplification ofthe pick-up and/or the amplification of the electronic amplifier isincreased, this is equivalent to compressing the scale along the aaxis.This, in turn, means that the characteristic will have a steeper slope,as shown by either the dasher line 68 and the phantom line 69',depending on how much the u-axis is compressed. And this, in turn,results in smaller response thresholds a and at respectively. Thus, itwill be seen that the greater the amplification, the smaller will be thenorth error. But, as explained above, the warmup time is greater.

Accordingly, there is provided, in accordance with a further feature ofthe present invention, an amplifier whose amplification is adjustable.When the gyrocompass is first turned on, the amplifier will be adjustedto produce at least approximately the already described aperiodicamplification, and this adjustment is maintained until the initialtransients have decayed, i.e., until the gyrocompass is stabilized inits quiescent state and a is below a predetermined threshold value. Theamplifier is then adjusted to the maximum amplification, as a result ofwhich the error angle 7 decreasesvirtually without transientoscillationsto the smallest possible value. This last-mentionedinterval, i.e., the time it takes for the gyrocompass to stabilize withits new amplifier setting, is negligibly small as compared to the timewhich it would have taken the gyrocompass to stabilize if the amplifierhad originally been adjusted to the high amplification value which,ultimately, produces the small north error.

In practice, the amplifier is adjusted automatically, i.e., changing thesetting of the amplifier from V to V can be effected by automatic means.Accordingly, means are provided which sense when a given threshold valueof a is reached and which, when this value is reached or when or isbelow this value, put out a signal which changes the adjustment of theamplifier from V to V The threshold value or can readily be selectedonce the transient characteristics of the system are known. If thesystem tends to overcontrol, the change-over from V to V may be delayedfor a certain time interval after the threshold value of a has beenreached.

FIGURE 4 is a block diagram of one embodiment of a circuit arrangementin which the amplification is in-. creased above V In FIGURE 4, blocks70 and 71 are to be considered as incorporating the correspondinglylegended components of FIGURE 2. Thus, block 7 0 represents the pick-upand amplifier, while block 71 represents the servomotor andtransmission. A signal a is applied to the input of block 71, shown atthe left, so that the output is Va. Block 70 is legended V(OL) toindicate that the amplification is variable. The amplifier is controlledby a motor 72 which adjusts the amplification value. This motor 72 isenergized by a source of electrical energy such as a battery 73, themotor 72 and the battery 73 being connected in circuit with a timedprogram switch 74. The signal at is also used to start the switch 74,the connection being such that the switch begins to operate when 0:drops below the value at which the system normally assumes itsstabilized quiescent state.

The circuit of FIGURE 4 operates as follows: when the gyrocompass isfirst put into operation, the amplifier unit 70 is adjusted to produceaperiodic amplification (V so that the compass will quickly assume itssta bilized state. There will, however, remain a certain north error.Once the system has reached its stabilized state, the elastic deflectionwill also have dropped to or below a given limit value. As a result, thetimed program switch is put into operation and the motor 72 then adjuststhe amplifier so as to increase the amplification value from V to apredetermined maximum value V if desired, the amplification can beincreased in a stepwise manner. In this way, the response thresholds ofthe servomotor unit of the follow-up system, depicted by block 71, movecloser together (see FIGURE 3) and the gyrocompass casing will alignitself with the earths axis more accurately.

FIGURE 5 is a block diagram of another embodiment of a circuitarrangement in which the amplification is increased above V Here, it isnot the signal at but the amplified signal Va which is taken olf betweenblocks 70 and 71 and which is used for controlling the change ofamplification. When a and hence Va have dropped below the predeterminedthreshold value, the signal Va actuates a relay 75 so as to energize theregulating motor 72.

The circuit of FIGURE 5 operates as follows: the block 70 is initiallyadjusted to produce the amplification V When VOL and a have fallen belowthe predetermined threshold value, the relay 75 switches on the motor 72and this increases the amplification. This increases Va and the relay isde-energized, thereby shutting off the motor 72. But as a continues todecrease, the relay will once more be energized, and this cyclecontinues until the maximum amplification V is reached.

It will be appreciated that while the amplifier control means have beenshown as being constituted by electromechanical components, electroniccomponents may readily be used instead.

It will also be seen from the above description that, thanks to the factthat the follow-up system incorporates an adjustable amplifier whoseamplification can be changed between V, and a greater amplification, thegyrocompass will rapidly reach its initial stabilized position, afterwhich the remaining north error can be reduced even further. There isthus obtained the advantage of increased accuracy without suffering thepenalty of prolonged warm-up time.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations.

I claim:

1. In a one-degree-of-freedom gyrocompass including a frame, a rotormounted in the frame for rotation about a spin axis, and an electricdrive motor for the rotor, a gyrocompass casing within which the frameis mounted, by a gas bearing, for rotation about a vertical axis,mechanical spring means elastically interconnecting the frame and thegyrocompass casing for opposing relative rotation between the two, saidspring means further being a means for supplying electrical energy tothe drive motor from an external source of electrical energy, pick-offmeans associated with the frame and the casing for producing an outputsignal proportional to the angle about the vertical axis between therotor spin axis and a predetermined diametral plane of the casing, anddriving means connected to rotate the casing about the vertical axis forreducing the angle between such diametral plane and the rotor spin axis,the improvement comprising a followup system having an input connectedto said pick-off means and an output connected to said driving means andincluding an amplifier for producing substantially aperiodicamplification where where 0=the moment of inertia of said rotor withrespect to said spin axis, w=th6 angular speed of said rotor,

w*=the angular speed of the earth, and =the geographic latitude of thelocation of the gyrocompass.

2. In a gyrocompass as defined in claim 1 said amplifier being variablebetween V and a value V greater than V and said follow-up system furthercomprising amplifier control means for increasing the amplification ofsaid amplifier above V during north alignment and after the angle abetween said frame and said gyrocompass casing has fallen below apredetermined value.

3. In a gyrocompass as defined in claim 2, said amplifier control meanscomprising a timed program switch for controlling the increase of theamplification of said amplifier.

4. In a gyrocompass as defined in claim 2, said amplifier control meansbeing connected to receive a signal which is a function of said angle abetween said frame and said gyrocompass casing for increasing theamplification as said angle or decreases.

5. In a gyrocompass as defined in claim 2, said amplifier control meansbeing connected to receive a signal which is a function of theamplification V times said angle a between said frame and saidgyrocompass casing.

References Cited UNITED STATES PATENTS 3,122,842 3/1964 Wrigley et a1.2,606,447 8/ 1952 Boltinghouse. 3,173,215 3/1965 Johnston. 3,194,6137/1965 Pierry et a1. 3,231,984 2/1966 Howe et a1.

ROBERT B. HULL, Primary Examiner.

US. Cl. X. R. 74--5.5

